1. Field of the Invention
The present application relates to refining and formation of refractory metals and, more specifically, to electrochemical reduction and purification of refractory metals, metal compounds, and semi-metals at low temperatures in non-aqueous ionic solvents using catalysts.
2. Description of the Related Art
The Kroll process and Hunter process are methods currently in use for the production of titanium metal from titanium dioxide. In these methods, TiO2 is reacted with chlorine gas to produce titanium tetrachloride, a volatile corrosive liquid. This is reduced to titanium metal by reacting with metallic magnesium in the Kroll process or with sodium in the Hunter process. Both processes are carried out at high temperatures in sealed reactors. Following this, a two-step refining process is carried out which includes two high temperature vacuum distillations to remove the alkali metal and its chloride from titanium metal.
The refining of titanium by electrochemical means has long been a sought after process. It has been shown in the literature that oxygen could be removed from titanium and titanium alloys using an electrochemical high temperature molten salt method. This has led to the development of a possible new method of extracting and refining titanium directly from the oxide ore and was published by G. Z. Chen, D. J. Fray and T. W. Farthing in Nature 407, 361 (2002), and PCT international application publication number WO 99/64638, 16 Dec. 1999. Both documents are incorporated herein by reference in their entirety. This process involves electrochemistry in a high temperature molten salt, molten CaCl2 at ˜800° C. In these publications, two different mechanisms are proposed for the reduction of titanium oxides. In the first mechanism, it is proposed that the Ca+2 ions are reduced to metallic Ca at the cathode. Then the Ca metal chemically reacts with the TiOx forming an oxygenated Ca species, CaO, which is soluble in the melt forming Ca+2 and O−2. The second mechanism proposed was the direct electrochemical reduction of the TiOx to Ti metal and an oxygen species such as O−2. This is followed by the migration of the O−2 to the carbon anode where it forms a volatile species such as CO or CO2.